This invention relates to a hydraulic brake system for automotive vehicles and, more particularly, to such a system having a master brake cylinder operable by a brake pedal, wheel brake cylinders connected to the master brake cylinder via one or several brake lines, a hydraulic power booster connected between the brake pedal and the master brake cylinder and supplied by a pressure medium source so as to boost the actuating force. The system also has a booster piston in a booster chamber and the pressurization of the booster piston is controllable by the brake pedal. Finally, the system has a valve arrangement which is connected into one brake line or into several brake lines and which is controllable by a brake slip control device by means of which the brake line can be blocked.
In hydraulic brake systems of this type each of the wheel brake cylinders is connected to the brake line via a solenoid valve which is open when de-energized (currentless) and to a return line, leading to an unpressurized reservoir, via a solenoid valve which is closed when de-energized (currentless). The solenoid valves are controllable by the brake slip control device. When a lock-up tendency appears at any one of the wheel brakes, the solenoid valves associated with its wheel brake cylinder are actuated by the brake slip control device, whereby, the connection from the wheel brake cylinder to the brake line is blocked. At the same time pressure medium is tapped from the wheel brake cylinder into the return line so that the pressure in the wheel brake cylinder will drop. So as to be able to subsequently build up the pressure again in the wheel brake cylinder, the solenoid valves are switched back and the wheel brake cylinder again will be separated from the return line and connected with the brake line. Now the wheel brake cylinder again will receive pressure medium from the brake line until the pressures will have adapted to one another and the control cycle will start over again. So as to make available the pressure medium amount required for such a control, in these brake systems the working chamber of the master brake cylinder or the brake line will be connected with the booster chamber via suitable valve arrangements at the onset of brake slip control. At the same time, a positioning sleeve will be pressurized which thereby will keep the booster piston and, hence, the pistons of the master brake cylinder, too, in a position in which the pistons of the master brake cylinder will still be able to effect a sufficient actuating stroke for carrying out an emergency braking operation.
These brake systems are disadvantageous in that the hydraulic power booster and the master brake cylinder as well as the wheel brake cylinders connected downstream of the master brake cylinder require the same pressure medium. Further, part of the brake system may fail when a solenoid valve connected between wheel brake cylinder and return line jams in its open position.